This invention relates generally to integrated circuits fabricated in silicon substrates, and more particularly, the invention relates to electrical interconnections and contacts in such integrated circuits.
It is well known that a multilayer integrated circuit interconnection structure is desirable to improve circuit performance as well as to increase circuit density. Prior art structures include etched contact holes formed in an oxide coated wafer surface, a layer of aluminum formed on the oxide surface and in the contact holes to form contact with selected regions of the wafer. The aluminum layer is patterned to interconnect specific portions of the integrated circuit. This contact and interconnection structure has not been especially satisfactory for many applications because the highly reactive aluminum pits the silicon and silicon oxide during high temperature processing reducing the structure's breakdown voltage. Subsequent processing steps such as passivation and multilayer interconnection increases the reaction between the aluminum and the silicon oxide. Elevated temperature required for some processing steps, on the order of 900.degree. C., further promotes aluminum and oxide reaction. The aluminum may recrystallize in formations which crack the insulating layer separating multiple metal layers. Thus, the use of aluminum contacts and interconnections severely restricts subsequent processing steps at elevated temperatures.
U.S. Pat. No. 4,042,953 issued to John H. Hall, applicant herein, for HIGH TEMPERATURE REFRACTORY METAL CONTACT ASSEMBLY AND MULTIPLE LAYER INTERCONNECT STRUCTURE discloses an ohmic contact and interconnect structure including a layer of refractory metal such as molybdenum between two layers of silicon. The bottom silicon layer provides an adherent interface with an underlying insulating layer such as silicon oxide or to contact surfaces of the substrate, while the top silicon layer is provided so that ohmic contact can be easily made to the refractory metal. While the multilayer structure is relatively inert and withstands subsequent high temperature processing, the contact structure can develop a high resistance after being exposed to higher temperatures. This is due to the fact that silicon in the contact holes is purified by reaction with the molybdenum to raise its resistance. There is also a problem with residual silicon dioxide on the surface of the silicon substrate in the contacting areas which causes erratic contact resistance.
The present invention is directed to an integrated circuit having improved high temperature contacts and interconnect structures which overcome problems associated with prior art structures.